[TheDong]

> But obviously, there’s no guarantee that the latency will always be that good

That guarantee already doesn't exist with RDS. The RDS master is in one AZ, your server may be in another (and if it isn't, well, RDS will fail over to another AZ eventually). The network latency between AWS AZs is usually good, but it can be arbitrarily high, up to full outages between AZs.

This is just a measure of degrees. Your application and/or business already has to handle some degree of network issues (aws network outages), so it's a tradeoff of whether the extra hops increase the chance enough to matter.